1. Field of the Invention
The invention is in the field of endondtic files.
2. Description of the Prior Art
Endodontic Files--Overview
In the middle of every tooth is a small tube, referred to as the root canal, approximately one inch long and varying in diameter from the diameter of a piece of angel hair spaghetti to the diameter of a human hair. This tube in normally filled with healthy tissue, but for a variety of reasons this tissue can become inflamed, infected and die. In order to remove the dead or dying tissue, the root canal must be "drilled out" or enlarged with special dental instruments until a clean tube is established that can be filled with a permanent hermetic material. The instruments used to accomplish this objective are called "endodontic files".
Prior to 1959, endodontic files were not manufactured according to any set standards. That is, the size, shape and overall configuration varied from manufacturer to manufacturer.
During 1959, a standard known as variable linear dimensional (VLD) standardization came into being. Endodontic files manufactured after 1959 were defined by the diameter of the file at a point approximately 1 mm from the tip (D1) as shown in FIG. 1. An endodontic file with a diameter of 0.10 m at D1 is designated as a number 10 instrument; an endodontic file with a diameter of 0.15 at D1 is designated as a number 15 instrument, etc. Endodontic files between a #6 and a #10 increase in size 0.02 mm per file size, from the #10 endodontic file to the #60 endodontic file, each one in the series increases by 0.05 mm, and after endodontic file #60 each one increases by 0.10 mm at D1. This sizing system has become the ISO (International Standards Organization) standard. The taper is also defined by the standard and is determined as follows. A point 15 mm from D1 is designated D2 as shown in FIG. 1. At D2, the diameter of each file is 0.30 mm greater than the diameter of the file at D1. There is a linear increase in diameter from D1 to D2 so that for each 1 mm increase in length, there is a 0.02 mm increase in diameter. It should be noted that the 15 mm height or D1-D2 distance and the 0.30 mm relationship between D1 and D2 is simply a standard or tradition and of little clinical significance. In other words, the D1-D2 distance and D1-D2 relationship could be other than 15 mm and 0.30 mm respectively and beneficial results could still be obtained.
During 1991 a new endodontic file sizing system known as constant percent (CP) came into being. The constant percent change sizing system was the first change in endodontic file size standardization in over thirty years and is based on changing the diameter of D1 by an exact percentage. The CP system came into being because for a myriad of clinical reasons, the diameter of many root canals has decreased since 1962. Accordingly, endodontic treatment became more and more difficult since the introduction of the variable linear dimensional standardization. Specifically, since endodontic instruments were used by hand or, as explained below, in oscillating tools, it was becoming increasingly difficult to treat the more difficult endodontic cases. The problem with the VLD system is that the percentage change from file size to file size is erratic under the VLD sizing system. For example, the percentage change of D1 from a #8 endodontic file to a #10 endodontic file is 20%, from a #10 endodontic file to a #15 endodontic file the change is 50%, and from a #15 endodontic file to a #20 endodontic file the change is 33%. In order to rectify this shortcoming, the CP change system was introduced and is based on a constant percent change from one file size to the next. Each D1 is exactly x% larger than the its predecessor, where x is chosen so that a root canal treatment is accomplished while minimizing the number of instruments needed to perform the root canal treatment. In this connection, in the currently utilized CP system, x is 29.17%. Additionally, files currently produced under the CP system utilize a D1 to D2 differential of 0.6 mm instead of 0.3 mm as is the case with files produced under the VLD system. The CP system seemingly provides a smoother transition from one file to the next since each file is the same percentage larger than its predecessor. A paper describing the CP system is entitled "Revolutionary New Concepts In Endodontic Instruments Sizing" by Herbert Schilder in Giornale Italiano di Endodonzia, Vol 7, No 4, 1993.
Tables 1 and 2 below shows the D1 and D2 diameters for a typical VLD and CP file system respectively, along with a percentage change of D1 diameters and the volume and percentage volume change between adjacent files between adjacent files in each system.
TABLE 1 ______________________________________ D1 VLD Diameter VLD Volume Incre- D1 % D2 VLD % ment diameter Change diameter Volume Change ______________________________________ 0.150 0.450 1.15 0.0500 0.200 33% 0.500 1.53 33% 0.0500 0.250 25% 0.550 1.97 29% 0.0500 0.300 20% 0.600 2.47 25% 0.0500 0.350 17% 0.650 3.03 23% 0.0500 0.400 14% 0.700 3.65 20% 0.0500 0.450 13% 0.750 4.33 19% 0.0500 0.500 11% 0.800 5.07 17% 0.0500 0.550 10% 0.850 5.86 16% 0.0500 0.600 9% 0.900 6.72 15% 0.1000 0.700 17% 1.000 8.60 28% 0.1000 0.800 14% 1.100 10.72 25% 0.1000 0.900 13% 1.200 13.08 22% 0.1000 1.000 11% 1.300 15.67 20% ______________________________________
TABLE 2 ______________________________________ D1 Constant Diameter Constant Constant Volume Incre- Pct D1 % Pct D2 Pct % ment diameter Change diameter Volume Change ______________________________________ 0.129 0.729 2.52 0.0380 0.167 29% 0.767 2.92 16% 0.0490 0.216 29% 0.816 3.49 19% 0.0630 0.279 29% 0.879 4.30 23% 0.0810 0.360 29% 0.960 5.49 27% 0.1050 0.465 29% 1.065 7.25 32% 0.1350 0.600 29% 1.200 9.90 37% 0.1750 0.775 29% 1.375 13.97 41% 0.2250 1.000 29% 1.600 20.26 45% ______________________________________
Also of note is that in 1993 endodontic files made of an alloy of nickel and titanium transformed the traditional methods of endodontic instrumentation and preparation. Endodontic files (referred to as reamers when used in a rotary mode in which case the flutes are duller than is the case with files which operate in a reciprocating mode) made from this alloy were, for the first time ever, capable of being used in a rotating dental drill. However, their use in an endodontic rotary mode created a new problem. As it turns out, rotating files or reamers can only predictably remove a specific volume of dentin (root material) per file pass. Since both prior art sizing systems (VLD and CP) remove irregular volumes per file size, as shown in Tables 1 and 2, clinical "workarounds" had to be employed in order to prepare root canals when using rotary files or reamers based on this geometry. For example, accordingly to one leading manufacturer's instructions, the reamers must be used repeatedly in a complicated protocol of alternating sizes in order to achieve the goal of preparing the root canal system when using the rotary technique.